Organic resins are used as optical components (such as optical lenses) in place of inorganic glasses based on the former's processability, light weight, lost cost, and impact resistance. The last few years, however, have seen the exposure of organic resins to high temperatures and high luminosities in association with the miniaturization of optical components and the increasing intensity of light sources. As a consequence, there is demand for the development of highly heat- and light-resistant transparent organic resins. It is widely known that silicone resins exhibit an excellent performance with regard, inter alia, to heat resistance, cold resistance, electrical insulation characteristics, weathering resistance, water repellency, and transparency, and curable silicone resin compositions are in use in a variety of industrial fields. Among these, their use as optical component materials can be contemplated because they are much more resistant to discoloration and physical deterioration than other organic resins.
Among curable silicone resin compositions, i.e., curable organopolysiloxane resin compositions, the addition-curable organopolysiloxane resin compositions are characterized by a rapid cure, by the absence of by-product generation during cure, and by an excellent moldability. As examples of such addition-curable organopolysiloxane resin compositions, Japanese Patent Publication (hereinafter referred to as “Kokoku”) No. Sho 52-44900 (44,900/1977), corresponding to U.S. Pat. No. 3,732,330) discloses a curable organopolysiloxane resin composition comprising methylphenylvinylpolysiloxane resin comprising C6H5SiO3/2 unit and CH2═CH(CH3)2SiO1/2 unit, di(dimethylhydrogensiloxy)diphenylsilane with the formula [H(CH3)2SiO]2Si(C6H5)2, and a platinum catalyst.
Kokoku No. Sho 53-20545 (20,545/1978) discloses a curable organopolysiloxane resin composition comprising methylphenylvinylpolysiloxane resin comprising monophenylsiloxane unit, diphenylsiloxane unit, dimethylsiloxane unit, and vinylmethylsiloxane unit; straight-chain methylphenylhydrogenpolysiloxane comprising diphenylsiloxane unit, methylhydrogensiloxane unit, dimethylsiloxane unit, and trimethylsiloxane unit; and a platinum catalyst. These compositions are useful as encapsulants, potting agents, and coatings for electrical and electronic components.
Japanese Patent Application Publication (hereinafter referred to as “Kokai”) No. 2002-265787 provides an addition-curable silicone resin composition comprising phenyl- and alkenyl-containing a organopolysiloxane resin, a phenyl-containing organohydrogenpolysiloxane, and a hydrosilylation curing catalyst; the cured product from this composition is transparent and exhibits high strength and high hardness and is useful for electrical and electronic instruments and devices, office automation equipment, and precision equipment and machinery.
However, methylphenylvinylpolysiloxane resins are generally solids or high-viscosity liquids, and for this reason the compositions referenced above do not exhibit good fluidity or good casting properties. Kokai No. Sho 54-159459 (159,459/1979) therefore provides an addition-curable organopolysiloxane resin composition that contains diphenylbis(dimethylvinylsiloxy)silane as a reactive diluent in addition to methylphenylvinylpolysiloxane resin comprising the C6H5SiO3/2 unit and CH2═CH(CH3)2SiO1/2 unit, dimethylhydrogensiloxyphenylsilane with the formula [H(CH3)2SiO]2Si(C6H5)2 or [H(CH3)2SiO]3Si(C6H5), and a platinum catalyst, while in its Example 1 Kokai No. Hei 11-1619 (1,619/1999) describes an addition-curable silicone resin composition that contains disiloxane with the formula [CH2═CH(CH3)2Si]2O in addition to a methylphenylvinylpolysiloxane resin, a methylphenylhydrogenoligosiloxane, and a platinum catalyst.
These patent references, however, neither describe nor suggest a curable organopolysiloxane resin composition or an addition-curable silicone resin composition that can give a cured product that is resistant to the phenomenon in which the entire cured product clouds up, with an attendant decline in light transmittance, when the cured product is cooled after being heated under high humidities. A cured article that has clouded up and suffered a decline in light transmittance is unqualified for use as an optical element. For example, with regard to such applications as an optical lens adjacent to a light-emitting element or as a molding material, sealant, coating, or adhesive for a light-emitting element or a light-receiving element, the reduction in light flux due to clouding of the cured material can cause defective operation or a decline in functionality. In addition, some of the preceding patent references suggest that a cured article with a hardness and strength satisfactory for practical applications is not obtained from addition-curable organopolysiloxane resin compositions that have a viscosity at 25° C. of less than 1 Pa·s.
The other patent references neither describe nor suggest a relationship between viscosity and hardness or strength. While a lower viscosity for an addition-curable organopolysiloxane resin composition is not necessarily always better, the viscosity is preferably as low as possible when the curing procedure involves, for example, spin coating or infiltration into a confined space under the action of an infiltration pressure. In the case of addition-curable organopolysiloxane resin compositions with a viscosity of 1 Pa·s or more, the additional requirement of, for example, solvent dilution or heating causes a reduction in processability.